In a typical push-in bulb assembly a bulb is mounted in a plastic lamp base and has a pair of lead wires which extend through the bottom of the lamp base and double back over opposite outer side faces of a plug-in portion of the lamp base. The lamp base plugs into a socket in a lampholder presenting contact elements at opposite sides which are engaged by the lead wires from the bulb. The contact elements in turn engage insulated wires which may be presented by a single cord as shown in the above-referenced patent and patent application. Preferably the plug-in portion of the lamp base and the socket in the lampholder are shaped so that the lamp base must be properly oriented for proper lead wire to contact element engagement in order for the plug-in portion to be pushed into the socket.
Sometimes the plug-in portion of the lamp base in a plug-in light assembly works loose from the lampholder socket sufficiently to break electrical contact between the bulb leads and the contact elements in the lampholder. This can occur, for example, during packing and shipping, or while the set is being handled while being mounted in a decorating position or removed therefrom for storage.
Although a relatively snug fit between the lamp base and the lampholder is preferred to maintain electrical contact between the bulb leads and the contact elements, it is also preferred to have the bulb holders relatively easy to remove for bulb replacement in case of bulb failure. Accordingly, there is a need for a practical, effective lock-on system to prevent unintentional dislodgement of the lamp bases in the lampholders of plug-in type decorative light units, but which can be easily manually released for bulb replacement. Since the lamp bases and lampholders are normally plastic injection-molded parts, a solution to the lamp loosening problem is not practical if it unduly complicates the molding process or assembly of the sets.